nucleus charge = +5 nucleus charge = +6 nucleus charge = +7 Boron Carbon Nitrogen

Transcription

1 ChemQuest 16 Name: Date: Hour: Information: Shielding FIGURE 1: Bohr Diagrams of boron, carbon and nitrogen nucleus charge = +5 nucleus charge = +6 nucleus charge = +7 Boron Carbon Nitrogen Because the nucleus is positively charged, it exerts an attractive force on the electrons. However, the three electrons in boron s outer energy level do not feel the full +5 attraction from the 5 protons in boron s nucleus. Before the +5 attraction gets to the outer energy level it gets partially cancelled (or shielded ) by the two electrons in the first energy level. The two electrons in the first energy level weaken the attractive force by two. Therefore to the outer energy level it only feels like a +3 charge rather than a +5 charge from the nucleus. Consider the diagram of carbon. An electron in the outer energy level only feels a charge of +4 coming from the nucleus because the two electrons in the first energy level shield two of the positive charges from the nucleus. 1. How large is the charge that the second energy level of nitrogen feels from the nucleus? 2. Why does the first energy level in each of the three above diagrams only contain two electrons? 3. How many electrons can fit in the second energy level of any atom? 4. How many electrons can fit in the third energy level? 5. How many energy levels does aluminum have? How many electrons should be in each energy level? 6. Draw a Bohr diagram for aluminum similar to those above.

2 7. Explain why the second energy level of aluminum only feels a +11 attraction instead of a +13 attraction from aluminum s electrons. 8. How large is the charge that the third energy level of an aluminum atom feels from the nucleus? Information: Charge and Distance As you know, opposite charges attract. Examine the following diagrams of charged metal spheres. FIGURE 2: Diagram A Diagram B Diagram C The attraction between the two charged metal spheres in each diagram is represented by an arrow. The metal spheres are pulled closer together in diagram C because of the +5 to -5 attraction is stronger than the +4 to -4 attraction in Diagram B and the +3 to -3 attraction in Diagram A. Electrons behave the same way as the metal spheres are depicted in Figure 2. Consider the Bohr diagrams of boron, carbon and nitrogen in Figure 1. Recall that Boron s outer electrons feel a +3 attraction from the nucleus. Carbon s outer electrons feel a +4 attraction. In question 1, you found out that nitrogen s outer electrons feel a +5 attraction. This attraction between the nucleus and outer energy level determines the size of the atom. If the attraction is strong the atom is small; if the attraction is weak, the atom spreads out and is larger. 9. Which atom is larger: nitrogen or carbon? Why? 10. In a silicon atom, the force of attraction from the nucleus to the outer energy level is +4. Using a Bohr diagram of a silicon atom as an illustration, explain in detail why this is true. 11. Draw Bohr diagrams for sulfur and chlorine.

3 12. a) Find the size of the charge attraction between the nucleus and outer energy level for sulfur and for chlorine. b) Which atom do you predict to be larger: sulfur or chlorine? c) Explain, in detail, your reasoning to part b. 13. Notice and compare the locations of boron, carbon and nitrogen on the periodic table. Now compare their sizes. Do the same with sulfur and chlorine. There is a general trend in size as you proceed from left to right across the periodic table. What is this trend? In other words, how do atoms in the same row of the periodic table compare to each other in size? Information: Bohr Diagrams and the Size of Atoms Examine the following Bohr Diagrams of three atoms from the periodic table. FIGURE 3 Atom A Atom B Atom C 14. Give the name and atomic number of each atom from Figure 3. Atom A Atom B Atom C Name of the element Atomic number 15. a) Concerning atoms A, B and C, what is similar about their location in the periodic table? b) In atoms A, B, and C compare the force of attraction from the nucleus to the outer level of electrons.

4 16. Draw Bohr diagrams for neon and argon. 17. a) What is similar about the location of neon and argon in the periodic table b) Compare the force of attraction between the outer level electrons and the nucleus for neon and argon. c) Using your answers to 15b and 17b, what can be said about elements in the same column and the force of attraction between their outer energy level and nucleus? 18. Atom A is larger than atom B. The attraction between the nucleus and outer level electrons is equal in atoms A and B, so what other reason could there be for Atom A s larger size? Propose an explanation based on that structure of atoms A and B. 19. Which is larger neon or argon? Why is this atom the largest? 20. In general, there is a trend in the sizes of atoms as you move down a column of the periodic table. a) What is this trend? b) Why does this trend exist? (Explain the basis for the trend.) 21. Order the following lists of elements in order from smallest to largest. a) K, As, Br b) P, Sb, N c) S, Ca, Mg, Cl

5 Skill Practice 16 Name: Date: Hour: 1. What force of attraction does the second energy level of a phosphorus atom feel from the nucleus? Draw a Bohr diagram and use it to explain your answer. 2. Using the concepts of shielding and attraction, explain why sulfur is smaller in radius than silicon. 3. Why can t you tell by looking at the periodic table whether chlorine or lithium is larger? 4. Order the following elements from smallest to largest. A) Al, Na, S, Mg B) C, Sn, Pb, Si C) K, Se, Ca, Br D) Be, Ca, C, B, Mg E) Ga, Al, Cl, P F) O, Se, S, Ne

6 ChemQuest 17 Name: Date: Hour: Information: Separating Charges Examine Figure 1 below where there are three pairs of metal spheres that have different amounts of charge on them. The spheres in diagram C are closer than the others because they have the strongest attraction. FIGURE 1: Attraction between metal spheres. Diagram A Diagram B Diagram C Which would be harder to separate: the two spheres in Diagram A or the two spheres in Diagram B? Why? 2. Draw Bohr diagrams of the following atoms: lithium, nitrogen and fluorine. 3. For each of the atoms in question two compare the attraction between the nucleus and the outer level of electrons. Which atom has the strongest attraction between the nucleus and outer electrons? 4. Which would be more difficult: if you wanted to remove an electron from an atom of fluorine or from an atom of nitrogen? 5. Would it be easier to remove an electron from lithium or nitrogen? Information: Ionization Energy The amount of energy that it takes to completely remove an electron from an atom is called ionization energy. The first ionization energy is the energy required to remove one electron from an atom s outer energy level. The second ionization energy is the energy needed to remove a second electron from the energy level. The second ionization energy is always higher than the first ionization energy. Because noble gases have eight electrons in their outer energy level, they are very stable and therefore it takes a very high amount of energy to remove an electron from a noble gas.

7 6. Which would have a higher first ionization energy: phosphorus or aluminum? (You may want to draw a Bohr diagram to help you determine the answer.) 7. a) Phosphorus and aluminum are in the same row of the periodic table. Lithium, nitrogen and fluorine are also in the same row. Using your answers to questions 3, 4 and 6 what do you notice about the ionization energy of elements proceeding from left to right across a row of the periodic table? Does the ionization energy increase or decrease as you go across a period? b) Explain why you believe this trend exists. 8. Draw a Bohr diagram of sodium. Do you expect the first ionization energy to be high or low for sodium? 9. The second ionization energy of all elements is higher than the first ionization energy, but in sodium the second ionization energy is extra large. Considering the structure of the sodium atom explain why this might be. (HINT: after one electron is removed, what is sodium s electron arrangement like?). 10. A certain atom in the third period has a third ionization energy that is unusually high. Using the same reasoning you used for question 9, name this atom. Information: Trend in Ionization Energy in Groups Consider the following figure of diagrams of two magnets that you wish to separate. FIGURE 2: Separating Magnets Diagram D Diagram E 11. Would it be easier to separate the magnets in Diagram D or those in Diagram E in Figure 2? Assume that each magnet is attracted to the other and that the size of the attraction is the same. Take only the distance between magnets into consideration.

8 12. How does the distance between the outer level of electrons and the nucleus in phosphorus compare to the distance between the outer level of electrons and the nucleus in nitrogen? 13. Based on your answer to question 12, would it be easier to remove an electron from nitrogen or from phosphorus? 14. Nitrogen and phosphorus are in the same column on the periodic table. From your answers to questions 12 and 13 what happens to the ionization energy as you move down a column in the periodic table?

9 Skill Practice 17 Name: Date: Hour: 1. If an atom has a high first ionization energy does this mean that it is relatively easy or relatively hard to remove an electron from the atom? 2. Arrange the following atoms in order from lowest to highest 1 st ionization energy. A) Ca, Se, As, Br B) As, N, P, Bi C) Ga, Al, S, Si D) Li, K, O, C E) Te, O, S, Po F) In, Te, Sn, I 3. A certain atom in the 2 nd period has an unusually high 3 rd ionization energy. Name this element. Draw a Bohr diagram and use it to illustrate why you were able to identify this atom. 4. Compare the trends for size and for ionization energy. As the size of an atom increases, what happens to the ionization energy? Explain why the ionization energy seems to depend on the size.